External-Resonator-Type Light Emitting Device

ABSTRACT

An external resonator type light emitting system includes a light source oscillating a semiconductor laser light by itself and a grating device providing an external resonator with the light source. The system performs oscillation in single mode. The light source includes an active layer oscillating the semiconductor laser light. The grating device includes an optical waveguide having an incident face to which the semiconductor laser is incident and an emitting face of emitting an emitting light of a desired wavelength, a Bragg grating formed in the optical waveguide, and a propagating portion provided between the incident face and the Bragg grating. Formulas (1) to (5) are satisfied.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an external resonator type lightemitting system.

2. Related Art Statement

It has been generally utilized a semiconductor laser of Fabry-Perot (FP)type including an optical resonator constructed with an active layer andmirrors provided on both end faces of the active layer. However,according to such FP type laser, light is oscillated at a wavelengthsatisfying conditions for oscillating standing waves. The longitudinalmode thus tends to be of multi mode, and the oscillating wavelength ischanged particularly when a current or temperature is changed, resultingin a change of optical intensity.

Therefore, for the purpose of optical communication or gas sensing, itis required a laser capable of single mode oscillation and with goodstability of wavelength. It has been thereby developed a distributedfeed-back (DFB) laser and a distributed reflection (DBR) laser.According to these laser systems, diffraction gratings are provided in asemiconductor material to oscillate light having only a specificwavelength utilizing the dependency of the gratings on wavelength.

According to the DBR laser, concaves and convexes are formed on asurface of a waveguide extended from a waveguide of the active layer toprovide a mirror utilizing Bragg reflection for realizing a resonator(Patent Document 1; Japanese Patent Publication No. S49-128,689A; PatentDocument 2; Japanese patent Publication No. S56-148,880A). Sincediffraction gratings are provided on both ends of the optical waveguidelayer according to the laser, light emitted from the active layer ispropagated through the optical waveguide layer, and a part of thepropagating light is reflected by the diffraction gratings, returnedinto a current injection part and then amplified. As light componenthaving only a single wavelength is reflected at a specific directionfrom the diffraction grating, the wavelength of the laser light is madeconstant.

Further, as the application, it was developed an external resonator typesemiconductor laser including a diffraction grating as a part separatedfrom the semiconductor to form an external resonator. Such type of laserprovides one having stability of wavelength, stability of temperatureand controllability. The external resonator includes a fiber Bragggrating (FBG) (Non-patent document 1) and a volume hologram grating(VHG) (Non-patent document 2). Since the diffraction grating is composedof a member separated from the semiconductor laser, it is characteristicthat its reflectance and length of the resonator can be independentlydesigned. And, since it is not affected by temperature rise due to heatgeneration caused by current injection, it is possible to furtherimprove the stability on wavelength. Further, the temperature dependencyof refractive index of the semiconductor is different, it is possible toimprove the stability on temperature by designing the refractive indextogether with the length of the resonator.

According to Japanese Patent document 6 (Japanese Patent Publication No.2002-134,833A), it is disclosed an external resonator type laserutilizing a grating formed in a waveguide composed of quartz glass. Itaims at providing a frequency-stable laser suitably used in environmentunder which room temperature is considerably changed, (for example, upto 30° C. or higher) without a temperature controller. It is furtherdescribed to provide a laser free from temperature dependency with modehopping prevented and without dependency of oscillating frequency ontemperature.

According to Patent document 7 (Japanese Patent Publication No.2010-171252A), it is disclosed an external resonator type laserincluding an optical waveguide composed of a core layer made of SiO₂,SiO_(1-x)N_(x) (x represents 0.55 to 0.65) or Si and SiN, and a gratingformed in the optical waveguide. This is an external resonator typelaser capable of maintaining the oscillation wavelength constantlywithout precise temperature control. For this, it is indispensable thatthe temperature dependence of the reflection wavelength of a diffractiongrating (temperature coefficient of the Bragg reflection wavelength) ismade low. It is further described that the longitudinal mode of thelaser oscillation is further made multi mode to realize stability ofpower.

According to Patent document 8 (Japanese patent No. 3667209B), it isdisclosed an external resonator type laser utilizing a grating formed inan optical waveguide made of quartz, InP, GaAs, LiNbO₃, LiTaO₃ orpolyimide resin. According to this, a reflectance at a light emittingface of a semiconductor laser as a light source is an effectivereflectance Re (substantially of 0.1 to 38.4%) and the longitudinal modeof the laser oscillation is made multi mode, so that stability of powercan be realized.

RELATED ARTS Non-Patent Documents

-   (Non-Patent document 1) “Transactions on Fundamentals of    Electronics, Communications and Computer Sciences” C-II Vol. J81,    No. 7 pp. 664-665, 1998 July-   (Non Patent document 2) “Technical Reports on Fundamentals of    Electronics, Communications and Computer Sciences” LQE, 2005, Vol    105, No. 52, pp. 17-20

Patent Documents

-   (Patent document 1) Japanese Patent Publication No. 549-128,689A-   (Patent document 2) Japanese Patent Publication No. 556-148,880A-   (Patent document 3) WO 2013/034,813-   (Patent document 4) Japanese Patent Publication No. 2000-082,864A-   (Patent document 5) Japanese Patent Publication No. 2006-222,399A-   (Patent document 6) Japanese Patent Publication No. 2002-134,833A-   (Patent document 7) Japanese Patent Publication No. 2010-171,252A-   (Patent document 8) Japanese Patent No. 3,667,209B

SUMMARY OF THE INVENTION

The non-patent document 1 refers to the mechanism of mode hoppingdeteriorating stability on wavelength accompanied with temperature riseand a method of solving it. An amount δλs of change of wavelength of anexternal resonator type laser depending on temperature is expressed asthe following formula based on standing wave condition, provided thatΔna is assigned to a change of refractive index of active layer regionof a semiconductor, La is assigned to a length of the active layer, Δnfand Lf are assigned to a change of refractive index and length,respectively, of FBG region, and δTa and δTf are assigned to changes oftemperatures of them, respectively.

$\begin{matrix}{{\delta \; \lambda_{s}} = {{\lambda_{0}\frac{\Delta \; n_{a}L_{a}}{{n_{f}L_{f}} + {n_{a}L_{a}}}\delta \; T_{a}} + {\lambda_{0}\frac{\Delta \; n_{f}L_{f}}{{n_{f}L_{f}} + {n_{a}L_{a}}}\delta \; T_{f}}}} & {{Formula}\mspace{14mu} (1)}\end{matrix}$

Here, λ0 represents a wavelength of reflection by a grating at initialstage.

Further, a change δλG of a wavelength of reflection of the grating isrepresented according to the following formula.

$\begin{matrix}{{\delta \; \lambda_{G}} = {\lambda_{0}\frac{\Delta \; n_{f}}{n_{f}}\delta \; T_{f}}} & {{Formula}\mspace{14mu} (2)}\end{matrix}$

Mode hopping is caused when a longitudinal mode spacing Δλ of theexternal resonator equals to a difference between the amount of changeof wavelength δλs and amount δλ_(G) of change of reflection wavelengthof the grating, so that the following formula is satisfied.

$\begin{matrix}{{\Delta \; \lambda} = {{\delta \; \lambda_{s}} - {\lambda_{0}\frac{\Delta \; n_{f}}{n_{f}}\delta \; T_{f}}}} & {{Formula}\mspace{14mu} (3)}\end{matrix}$

The longitudinal mode spacing Δλ is represented approximately accordingto the following formula.

$\begin{matrix}{{\Delta \; \lambda} = \frac{\lambda_{0}^{2}}{2\left( {{n_{f}L_{f}} + {n_{a}L_{a}}} \right)}} & {{Formula}\mspace{14mu} (4)}\end{matrix}$

Formula (5) is satisfied based on the formulas (3) and (4).

$\begin{matrix}{{\Delta \; T_{all}} = \frac{\lambda_{0}}{2n_{a}{L_{a}\left( {{\Delta \; {n_{a}/n_{a}}} - {\Delta \; {n_{f}/n_{f}}}} \right)}}} & {{Formula}\mspace{14mu} (5)}\end{matrix}$

For preventing the mode hopping, it is necessary to use within atemperature range smaller than ΔTall, so that the temperature iscontrolled by using a Pertier device. According to the formula (5), inthe case that the changes of the reflective indices of the active layerand grating layer are the same (Δna/na=Δnf/nf), the denominator becomeszero and the temperature for causing the mode hopping becomes infinitevalue, indicating that the mode hopping would not occur. According to amonolithic type DBR laser, however, since current is injected into theactive layer for laser oscillation, the changes of the refractiveindices of the active layer and grating layer cannot be matched witheach other, resulting in the mode hopping.

The mode hopping is the phenomenon that the oscillating mode(longitudinal mode) within the resonator is shifted from one mode toanother mode. As the temperature or injection current is changed, theconditions of the gain and resonator are changed and the wavelength ofthe oscillated laser is thereby changed, resulting in the problem,called kink, that the optical power is deviated. In the case of an FPtype GaAs semiconductor laser, therefore, the wavelength is normallychanged at a temperature coefficient of 0.3 nm/° C., and moreconsiderable deviation would occur when the mode hopping takes place. Atthe same time, the output is changed by 5 percent or more.

Therefore, for preventing the mode hopping, a Pertier device is used forcontrolling the temperature. A number of parts are thereby increased,the size of a module is enlarged and its cost is made high.

According to the patent document 6, for preventing the dependency ontemperature, the structure of a prior resonator itself is maintained anda stress is applied on an optical waveguide layer to compensate atemperature coefficient due to the thermal expansion to realize thenon-dependency on temperature. Therefore, a metal plate is adhered ontothe device and it is further added a layer of adjusting the temperaturecoefficient within the waveguide. There is a problem that the resonatorstructure is further enlarged.

An object of the present invention is to reduce the mode hopping, toimprove the stability on wavelength and to reduce the deviation of anoptical power, without using a Peltier device.

The present invention provides an external resonator type light emittingsystem comprising a light source oscillating a semiconductor laser lightby itself and a grating device providing an external resonator with thelight source, the system performing oscillation in single mode;

wherein the light source comprises an active layer oscillating thesemiconductor laser light;

wherein the grating device comprises an optical waveguide comprising anincident face for making the semiconductor laser light incident and anemitting face of emitting an emitting light having a desired wavelength,a Bragg grating formed in the optical waveguide, and a propagatingportion provided between the incident face and the Bragg grating; andwherein the following formulas (1) to (5) are satisfied.

$\begin{matrix}{{\Delta \; \lambda_{G}} \geqq {0.8\mspace{14mu} {nm}}} & (1) \\{L_{b} \leqq {500\mspace{14mu} {µm}}} & (2) \\{L_{a} \leqq {500\mspace{14mu} {µm}}} & (3) \\{n_{b} \geqq 1.8} & (4) \\{{{\frac{\lambda_{G}}{T} - \frac{\lambda_{TM}}{T}}} \leqq {0.03\mspace{14mu} {{nm}/{{{^\circ}C}.}}}} & (5)\end{matrix}$

(Δλ_(G) represents a full width at half maximum of a peak of a Braggreflectance in the formula (1).

L_(b) represents a length of the Bragg grating in the formula (2).

L_(a) represents a length of the active layer in the formula (3).

n_(b) represents a refractive index of a material forming the Bragggrating in the formula (4).

dλ_(G)/dT represents a temperature coefficient of a Bragg wavelength,and dλ_(TM)/dT represents a temperature coefficient of a wavelengthsatisfying a phase condition of an external resonator laser in theformula (5).)

Generally in the case that a fiber grating is used, as quartz is low ina temperature coefficient of a refractive index and dλ_(G)/dT is thuslow and |dλ_(G)/dT−dλ_(TM)/dT| becomes larger. The temperature range ΔTin which the mode hopping occurs becomes narrow so that the mode hoppingtends to take place.

According to the present invention, it is used a material having arefractive index of 1.8 or higher for an optical waveguide in which agrating is formed. It is thus possible to increase the temperaturecoefficient of the refractive index and dλ_(G)/dT, so that |dλ_(G)/dT−dλ_(TM)/dT| can be made lower. It is thereby possible to enlarge thetemperature range ΔT in which the mode hopping takes place.

According to a preferred embodiment, the relationship of the followingformulas (6) to (8) are satisfied.

L _(WG)≦600 μm  (6)

1 μm≦L _(g)≦10 μm  (7)

20 μm≦L _(m)≦100 μm  (8)

(L_(WG) represents a length of the grating device in the formula (6).

L_(g) represents a distance between an emitting face of the light sourceand the incident face of the optical waveguide layer in the formula (7).

L_(m) represent a length of the propagating portion in the formula (8).)

According to the present invention, it is possible to reduce the modehopping, to improve the stability on wavelength and to reduce thedeviation of the optical power without the use of a Peltier device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view schematically showing an external resonator type lightemitting system.

FIG. 2 is a cross sectional view of a grating device.

FIG. 3 is a perspective view schematically showing the grating device.

FIG. 4 is a cross sectional view showing another grating device.

FIG. 5 is a diagram for illustrating pattern of mode hopping accordingto a prior art.

FIG. 6 is a diagram for illustrating pattern of mode hopping accordingto a prior art.

FIG. 7 is a diagram for illustrating pattern of mode hopping accordingto the inventive example.

FIG. 8 shows spectrum of light amount of a light source and spectrum ofa system obtained by adding a grating device to the light source,according to example 2.

FIG. 9 shows reflection characteristics (gain condition) and phasecondition in a prior structure.

FIG. 10 shows reflection characteristics (gain condition) and phasecondition in a structure of the invention.

FIGS. 11(a), 11(b) and 11(c) are diagrams schematically showing crosssections of grating devices 21A, 21B and 21C, respectively, utilizingelongate strip-shaped optical waveguides 30, 30A.

FIGS. 12(a) and 12(b) are diagrams schematically showing grating devices21D and 21E, respectively, utilizing elongate and stripe-shaped opticalwaveguides 30 and 30A.

EMBODIMENTS OF THE INVENTION

An external resonator type light emitting system 1, schematically shownin FIG. 1, includes a light source 1 oscillating a semiconductor laserlight, and a grating device 9. The light source 2 and grating device 9are mounted on a common mount 3.

The light source 2 includes an active layer 5 oscillating thesemiconductor laser light. According to the present embodiment, theactive layer 5 is provided on a substrate 4.

Here, as the light source 2, it is used a light source capable ofoscillating laser by itself. It means that the light source 2 oscillateslaser by itself without the need of a grating device.

It is preferred that the light source 2 oscillates laser by itself inlongitude mode and single mode. However, in the case of an externalresonator type laser utilizing a grating device, the refractive propertycan be made dependent on wavelength. By controlling the pattern of thedependence on the wavelength, it is possible that the external resonatortype laser can oscillate laser in single mode, even when the lightsource 2 oscillates by itself laser in longitudinal mode and in multimode.

A high reflection film 6 is provided on an outer end face of thesubstrate 4, and a reflection layer 20 is formed on an end face of theactive layer 5 on the side of the grating device.

As shown in FIGS. 1 and 3, in the grating device 9, it is provided anoptical material layer 11 including an incident face 11 b, to which asemiconductor laser light A is incident, and an emitting face 11 b ofemitting emission light B of a desired wavelength. C representsreflected light. A Bragg grating 12 is formed in the optical materiallayer 11. A propagating portion 13 without a diffraction grating isprovided between the incident face 11 a of the optical material layer 11and the Bragg grating 12, and the propagating portion 13 opposes to theactive layer 5 through a spacing 14. 7B represents an antireflectionfilm provided on the side of the incident face of the optical materiallayer 11, and 7C represents an antireflection film provided on the sideof the emitting face of the optical material layer 11. According to thepresent example, the optical material layer 11 is of a ridge typeoptical waveguide, and provided on a substrate 10. The optical materiallayer 11 may be formed on the same face or on the opposing face as theBragg grating 12.

According to a preferred embodiment, the reflectance value of the Bragggrating is larger than those at the light emitting end of the lightsource, at the light incident face of the grating device and at thelight emitting face of the grating device. On the viewpoint, thereflectance values, at the light incident face of the grating device andat the light emitting face of the grating device may preferably be 0.1percent or lower. It is thus preferred that antireflection layers 7B and7C are formed on the incident and emitting faces of the grating device.The reflectance of the antireflection layer is lower than thereflectance of the grating and may preferably be 0.1 percent or lower.However, in the case that the reflectance at the end face is lower thanthe reflectance of the grating, the antireflection film is not necessaryand a reflective film may be used.

As shown in FIG. 2, according to the present example, an opticalmaterial layer 11 is formed on the substrate 10 through an adhesivelayer 15 and a lower buffer layer 16, and an upper buffer layer 17 isformed on the optical material layer 11. For example, a pair of ridgegrooves 19 is formed in the optical material layer 11, and a ridge typeoptical waveguide 18 is formed between the ridge grooves. In this case,the Bragg grating may be formed on a flat face 11 a or face 11 b. On theviewpoint of reducing deviation of shape of the Bragg grating or ridgegroove, the Bragg grating is preferably formed on the face 11 a so thatthe Bragg grating and ridge grooves 19 are provided on the oppositesides of the substrate.

Further, according to a device 9A shown in FIG. 4, the optical materiallayer 11 is formed on the substrate 10 through the adhesive layer 15 andlower buffer layer 16, and the upper buffer layer 17 is formed on theoptical material layer 11. For example a pair of the ridge grooves 19 isformed on the side of the substrate 10 in the optical material layer 11,and the ridge type optical waveguide 18 is formed between the ridgegrooves 19. In this case, the Bragg grating may be formed on the side ofthe flat face 11 a or on the face 11 b with the ridge grooves formed. Onthe viewpoint of reducing the deviation of shape of the Bragg grating orridge groove, the Bragg grating may preferably be formed on the side ofthe flat face 11 a so that the Bragg grating and ridge grooves 19 areprovided on the opposite sides of the substrate. Further, the upperbuffer layer 17 may be omitted, and in this case, air layer can bedirectly contacted with the grating. It is thus possible to increase adifference of the refractive indices in the cases that the gratinggroove is present and absent, so that it is possible to make thereflectance larger with the grating having a smaller length.

In this case, the oscillation wavelength of the laser beam is decided onthe wavelength reflected by the grating. In the case that the lightreflected by the grating and the light reflected at the end face of theactive layer 5 on the side of the grating device exceed the thresholdvalue of the laser gain, it is satisfied the oscillation condition. Itis thus possible to obtain laser beam having high stability ofwavelength.

For further improving the stability of wavelength, it is effective thatloop gain from the grating is made larger. On the viewpoint, it ispreferred that the reflectance of the grating is larger than thereflectance at the end face of the active layer 5.

As the light source, it is preferred a laser of a GaAs series materialor InP series material having high reliability. As an application of theinventive structure, for example, in the case that a non-linear opticaldevice is utilized to oscillate green-light laser as a second harmonicwave, it is to be used laser of GaAs series oscillating at a wavelengtharound 1064 nm. As the reliability of the GaAs series or InP serieslaser is excellent, it is possible to realize a light source such as alaser array or the like composed of lasers arranged one-dimensionally.As the wavelength becomes longer, the temperature dependence of theBragg wavelength becomes larger. It is thus particularly preferred thatthe oscillation wavelength of the laser is 990 nm or lower for improvingthe stability of the wavelength. On the other hand, as the wavelengthbecomes shorter, the change Δna of the refractive index of thesemiconductor becomes too larger. It is thus particularly preferred thatthe oscillation wavelength of the laser is 780 nm or higher forimproving the stability of the wavelength. Further, materials andwavelength of the active layer may be appropriately selected.

The ridge type optical waveguide may be obtained by physicallyprocessing and shaping with, for example, grinding by means of aperipheral cutting edge or laser ablation.

The buffer layer may function as the clad layer for the opticalwaveguide. On the viewpoint, the refractive index of the buffer layermay preferably be lower than that of the optical material layer, and thedifference of the refractive indices may preferably be 0.2 or larger andmore preferably be 0.4 or larger.

The Bragg grating may be formed by physical or chemical etching asfollows.

Specifically, a metal film such as Ni or Ti is formed on the substrateof a high refractive index, and windows are periodically formed byphotolithography to form a etching mask. Thereafter, a dry etchingsystem, such as a reactive ion etching system, is utilized to form thegrating grooves periodically. At last, the metal mask is removed to formit.

In the optical material layer, for further improving the resistanceagainst optical damage of the optical waveguide, it may be contained oneor more metals selected from the group consisting of magnesium (Mg),zinc (Zn), scandium (Sc) and indium (In), and in this case, magnesium ismost preferred. Further, in the crystal, it may be contained a rareearth element as a dopant. The rare earth element may preferably be Nd,Er, Tm, Ho, Dy or Pr.

The material of the adhesive layer may be an inorganic adhesive, or anorganic adhesive, or a combination of the inorganic and organicadhesives.

Further, the optical material layer 11 may be formed by a film-formingmethod on a supporting body. Such film-forming method includessputtering, vapor deposition or CVD. In this case, the optical materiallayer 11 is directly provided on the supporting body and the abovedescribed adhesive layer is not present.

Further, the thickness of the optical material layer may more preferablybe 0.5 to 3.0 μm.

Specific material of the supporting body is not particularly limited,and includes lithium niobate, lithium tantalate, a glass such as quartzglass, quartz, Si or the like.

The reflectance of the antireflective film is necessarily lower than thereflectance of the grating. The material of the film forming theantireflective film includes a film formed by lamination of oxides suchas silicon dioxide, tantalum pentoxide or the like and a metal.

Further, the end faces of the light source device and grating device maybe cut in an inclined direction for reducing the reflection at the endfaces, respectively. Further, although the grating device and supportingbody may be joined by fixing by adhesion according to the example ofFIG. 2, they may be directly joined with each other.

The meaning of the conditions defined in the formulas (1) to (8) will befurther described below.

As mathematical formulas are abstract and difficult to understand,however, typical embodiments of a prior art and the present inventionwill be directly compared with each other first to describe thecharacteristics of the present invention. The conditions of the presentinvention will be then described.

First, condition for oscillating a semiconductor laser is decided on thegain condition and phase condition, as the following formula.

(C _(out) ²)⁴ |r ₁ ∥r ₂|exp{(ζ_(t) g _(th)−α_(a))L _(a)−α_(b) L_(b)}×exp{j(−φ₁−φ₂−2βL _(a))}=1   (2-1)

The gain condition is expressed as the following formula from theformula (2-1).

$\begin{matrix}{{\zeta_{t}g_{th}} = {{\alpha_{a}L_{a}} + {\alpha_{b}L_{b}} + {\frac{1}{L_{a}}{\ln \left( \frac{1}{{r_{1}}{r_{2}}C_{out}^{2}} \right)}}}} & {{Formula}\mspace{14mu} \left( {2\text{-}2} \right)}\end{matrix}$

Besides, α_(a) and α_(b) are loss coefficients of the active layer andgrating layer, respectively, La and Lb are lengths of the active layerand grating layer, respectively, r1 and r2 are reflectances of a mirror(r2 represents a reflectance of the grating), Cout represents aconnection loss of the grating device and light source, ξ_(t)g_(th)represents a gain threshold value of a laser medium, φ1 represents anamount of change of phase due to a reflection mirror on the side of thelaser, and φ2 represents an amount of change of phase in the gratingportion.

The formula (2-2) indicates that laser oscillation occurs in the casethat the gain ξ_(t)g_(th) (gain threshold value) of the laser mediumexceeds the loss. The gain curve (dependency on wavelength) of the lasermedium provides a full width at half maximum of 50 nm or larger andexhibits broad characteristics. Further, the loss part (right column)shows hardly any dependency on wavelength other than the reflectance ofthe grating, so that the gain condition is decided on the grating. As aresult, as shown in the comparison table, the gain condition can beevaluated only by the grating.

On the other hand, the phase condition is as shown in the followingformula from the formula (2-1). However, φ1 becomes zero.

φ₂+2βL _(a)=2pπ (p represents an integer)   Formula (2-3)

In the case that the light source 2 oscillates laser, compositeresonator mode is provided and the formulas (2-1), (2-2) and (2-3)described above are modified to complex formulas, respectively, However,they may be considered as standards of the laser oscillation.

As to the external resonator type laser, it has been commercializedthose utilizing an external resonator including quartz glass opticalwaveguide or FBG. According to prior design concept, as shown in table 1and FIGS. 5 and 6, the reflection characteristic of the grating was Δλgof about 0.2 nm and a reflectance of 10 percent. The length of thegrating portion was thereby made 1 mm. On the other hand, as to thephase condition, the wavelength satisfying it takes discrete values andit is designed so that the formula (2-3) is satisfied at two or threepoints within Δλg. It becomes thereby necessary the active layer of thelaser medium whose length is large, and it has been used the activelayer having a length of 1 mm or larger.

TABLE 1 Prior structure Present invention Reflection FIG. 9 FIG. 10Characteristics (gain condition) and Phase condition Material FBG, LN(used for glass waveguide ordinary light), GaAs, Ta2O5 ZnO, Al2O3 Lengthof grating Comparative Example: 100 μm Lb Example: 1 mm Length of LDComparative Example: 300 μm Active layer Example; 2.1 mm Mode hoppingComparative Example: 60° C. Temperature Example; 5° C. (operationaltemperature range) Change of wavelength 0.01 nm/° C. 0.05 nm/° C.Deviation of power Deviation of power 3% or smaller by mode hopping; 5%or larger Notes Temperature control Peltier device is not with Peltierdevice is needed needed

In the case of a glass waveguide or FBG, the dependency of λg ontemperature is very small, and dλ_(G)/dT becomes about 0.01 nm/° C. As aresult, the external resonator type laser has the characteristics ofstability on wavelength.

Contrary to this, the dependency of the wavelength satisfying the phasecondition on temperature is large and dλ_(G)/dT is 0.05 nm/° C., and thedifference reaches 0.04 nm/° C.

Further, in the case that the core layer is composed of SiO₂ orSiO_((1-x))N_(x), the change rate Δnf of the refractive index ontemperature is as low as 1×10⁻⁵/° C., and the change rate of λg ontemperature is very low at a wavelength of 1.3 μm and dλ_(G)/dT becomes0.01 nm/° C. On the other hand, as to the temperature coefficient of thewavelength (oscillation wavelength) at which the phase matchingcondition of the external resonator is satisfied, in the case that anInGaAsP type laser is used, and provided that an equivalent refractiveindex of the light source is 3.6, the change rate of the refractiveindex on temperature is 3×10⁻⁴/° C., the length La is 400 μm, theequivalent refractive index of the grating is 1.54, the change rate is1×10⁻⁵/° C., and the length is 155 μm, dλ_(G)/dT becomes 0.09 nm/° C.The difference becomes, therefore, 0.08 nm/° C.

The waveform of the spectrum of the laser beam thus oscillated has aline width of 0.2 nm or smaller. On the viewpoint of enabling laseroscillation in a wide temperature range and making a temperature rangefree from the mode hopping broader, it is preferred that the laseroscillation wavelength of the external resonator at room temperature(25° C.) is shorter than the central wavelength of the gratingreflectance. In this case, as the temperature rises, the laseroscillation wavelength is shifted to a longer wavelength side, so thatlaser is oscillated at a wavelength longer than the central wavelengthof the grating reflectance.

Further, on the viewpoint of enabling the laser oscillation in a widetemperature range and of enlarging the temperature range free from themode hopping, it is preferred that the laser oscillation wavelength ofthe external resonator at room temperature (25° C.) is longer than theoscillation wavelength of the light source 2 at the same temperature. Inthis case, as the temperature rises, the laser oscillation wavelength ofthe external resonator becomes shorter than the oscillation wavelengthof the light source 2.

The difference between the laser oscillation wavelength of the externalresonator and the oscillation wavelength of the light source 2 at roomtemperature may preferably be 0.5 nm or larger and further may be 2 nmor larger, on the viewpoint of enlarging the temperature rangepermitting the laser oscillation. However, if the difference of thewavelengths becomes too larger, the temperature dependence of the powerbecomes large. On the viewpoint, the difference may preferably be 10 nmor smaller and more preferably be 6 nm or smaller.

Generally, the temperature Tmh at which the mode hopping takes place canbe considered as the following formula based on the non-patent document1 (It is provided that Ta=Tf).

ΔG_(TM) is a spacing (longitudinal mode spacing) of the wavelengthssatisfying the phase condition of the external resonator type laser.

$\begin{matrix}{T_{mh} = \frac{\Delta \; G_{TM}}{{\frac{\lambda_{G}}{T} - \frac{\lambda_{TM}}{T}}}} & {{Formula}\mspace{14mu} \left( {2\text{-}4} \right)}\end{matrix}$

As a result, Tmh becomes about 5° C. according to a prior art, so thatit is susceptible to the mode hopping. In the case that the mode hoppingoccurs, the power is deviated based on the reflection characteristics ofthe grating by 5 percent or more.

As described above, in actual operation, a Pertier device has been usedto perform temperature control in the prior external resonator typelaser utilizing the glass waveguide or FBG.

Contrary to this, the present invention utilizes the grating device inwhich the denominator of the formula (2-4) becomes small as aprecondition. It is needed that the denominator of the formula (2-4) ismade 0.03 nm/° C. or lower, and specific material may preferably begallium arsenide (GaAs), lithium niobate (ordinary light), tantalumoxide (Ta₂O₅), zinc oxide (ZnO), or aluminum oxide (Al₂O₃). For example,in the case that lithium niobate (ordinary light) is used, that Δλ_(G)is designed at about 1.3 nm and that the length of the active layer ismade 250 μm for making two wavelengths satisfying the phase conditionare present within Δλ_(G), ΔG_(TM) becomes 1.2 nm and Tmh becomes 60° C.for example, so that it is possible to enlarge the operationaltemperature range. FIG. 7 shows this example.

In the case that the wavelengths satisfying the phase condition arepresent at five or less points within Δλ_(G), the mode hopping can beprevented, so that the operation can be made at stable laser oscillatingcondition, and the oscillation takes place with longitudinal mode assingle mode. The spectrum width of the output of the laser oscillationunder the condition becomes 0.1 nm or lower.

That is, according to the inventive structure, although the oscillatingwavelength is changed at 0.05 nm/° C. based on the temperaturecharacteristics of the grating with respect to the temperature change,it is possible to make the mode hopping difficult to take place.According to the inventive structure, the length Lb of the grating ismade 100 μm for enlarging Δλ_(G), and La is made 250 μm for enlargingλG_(TM).

Besides, the difference over the patent document 6 is supplemented.

The present invention is to realize the non-dependency on temperature bymaking the temperature coefficient of the wavelength of the gratingcloser to the temperature coefficient of the longitudinal mode, so thatit is possible to make the resonator structure compact without thenecessity of an additional part. According to the patent document 6,each parameter is described as follows, which is within a range of aprior art.

Δλ_(G)=0.4 nm

Spacing of longitudinal mode λG_(TM)=0.2 nm

Length of grating Lb=3 mm

Length of LD active layer=600 μm

Length of propagating portion=1.5 mm

Each condition of the present invention will be described further below.

The full width at half maximum Δλ_(G) in a peak of Bragg reflectance ismade 0.8 nm or higher (formula 1). λ_(G) represents Bragg wavelength.That is, as shown in FIGS. 5, 6 and 7, in the case that the horizontalaxis is assigned to the reflection wavelength by the Bragg grating andthe vertical axis is assigned to the reflectance, the wavelength atwhich the reflectance takes the maximum is assigned to the Braggwavelength. Further, in the peak whose center is positioned at the Braggwavelength, a difference of two wavelengths at which the reflectancetakes a half of the peak maximum is assigned to a full width at halfmaximum Δλ_(G).

The full width at half maximum Δλ_(G) at the peak of the Braggreflectance is made 0.8 nm or larger so that the peak of the reflectanceis made broad as shown in FIG. 7. On the viewpoint, the full width athalf maximum Δλ_(G) may preferably be made 1.2 nm or larger and morepreferably be made 1.5 nm or larger. Further, full width at half maximumΔλ_(G) may preferably be made 5 nm or smaller, 3 nm or smaller and 2 nmor smaller.

The length Lb of the Bragg grating is made 500 μm or smaller (formula2). The length Lb of the Bragg grating is a length of the grating in thedirection of an optical axis of light propagating in the opticalwaveguide. It is a precondition of the inventive design concept toshorten the length Lb of the Bragg grating to 500 μm or smaller, whichis shorter than that in a prior art. On the viewpoint, the length Lb ofthe Bragg grating may preferably be made 300 μm or smaller. Further, Lbmay more preferably be made 200 μm or smaller.

The length La of the active layer is also made 500 μm or smaller(formula 3). It is also a precondition of the inventive design conceptto shorten the length La of the active layer than that in a prior art.On the viewpoint, the length La of the active layer may preferably bemade 300 μm or smaller. Further, the length La of the active layer maypreferably be made 150 μm or larger.

The refractive index n_(b) of a material forming the Bragg grating ismade 1.8 or higher (formula 4). According to a prior art, it has beengenerally used a material having a lower refractive index such asquartz. According to the concept of the present invention, therefractive index of the material forming the Bragg grating is madehigher. The reason is that the material having a larger refractive indexprovides a larger dependency of the refractive index on temperature, andthat Tmh of the formula (2-4) can be made larger, and that thetemperature coefficient dλ_(G)/dT of the grating can be made larger. Onthe viewpoint, n_(b) may more preferably be 1.9 or higher. Further,although the upper limit of n_(b) is not particularly defined, it maypreferably be 4 or lower because the formation of the grating may bedifficult in the case that the grating pitch is too small. nb may morepreferably be 3.6 or lower. Further, on the same viewpoint, theequivalent refractive index of the waveguide may preferably be 3.3 orlower.

In addition to this, the condition defined in the formula (5) isimportant.

In the formula (5), dλ_(G)/dT represents a temperature coefficient ofthe Bragg wavelength.

Further, dλ_(TM)/dT represents a temperature coefficient of wavelengthsatisfying the phase condition of the external resonator laser.

Here, λ_(TM) represents a wavelength satisfying the phase condition ofthe external resonator laser, that is, the wavelength satisfying thephase condition of the formula (2-3) as described above. This is called“longitudinal mode” in the specification.

The longitudinal mode will be supplemented below.

Since φ2+2βLa=2pπ and β=2π/λ according to the formula (2-3), λsatisfying them is assigned to λ_(TM). φ2 represents a change of phaseof the Bragg grating, and is calculated according to the followingformula.

$r_{2} = {\frac{{- j}\; \kappa \mspace{11mu} {\tanh \left( {\gamma \; L_{b}} \right)}}{\gamma + {\left( {{\alpha/2} + {j\; \delta}} \right){\tanh \left( {\gamma \; L_{b}} \right)}}} \equiv {{r_{2}}{\exp \left( {{- j}\; \varphi} \right)}}}$

λG_(TM) represents a spacing (longitudinal mode spacing) of thewavelengths satisfying the phase condition of the external resonatorlaser. Since a plurality of λ_(TM) are present, it means a difference ofa plurality of λ_(TM). Δλ previously used is equal to λG_(TM) and λs isequal to λ_(TM).

Therefore, by satisfying the formula (5), it is possible to make thetemperature of mode hopping higher to prevent the mode hopping in apractical view. The numerical value of the formula (5) may morepreferably be made 0.025 or lower.

The length L_(WG) of the grating device is made 600 μm or smaller(formula 6). It is also a precondition of the present invention toshorten it as Lb. On the viewpoint, LWG may preferably be 400 μm orsmaller and more preferably be 300 μm or smaller. Further, LWG maypreferably be 50 μm or larger.

The distance Lg between the emitting face of the light source andincident face of the optical waveguide is made 1 μm or larger and 10 μmor smaller (formula 7). The stable oscillation can thereby be realized.

The length Lm of the propagation portion is made 20 μm or larger and 100μm or smaller (formula 8). The stable oscillation can thereby berealized.

According to a preferred embodiment, the light source and grating deviceare directly and optical coupled to each other, the Bragg grating andthe outer end face opposite to the emitting face of the active layerconstitute a resonator structure, and a length between the outer sideend face of the active layer and the end point on emitting side of theBragg grating is 900 μm or smaller. As light is gradually reflected inthe grating portion, it is not possible to observe the reflection pointclearly as a reflective mirror. Although the effective reflection pointcan be mathematically defined, it is present on the side of the laserwith respect to the end point of the Bragg grating on the emitting side.Considering this, according to the present invention, the length of theresonator is defined at the end point on the emitting side. According tothe present invention, even when the length of the resonator is veryshort, it is possible to oscillate light of a target wavelength at ahigh efficiency. On the viewpoint, the length between the outer side endface of the active layer and the end point of the Bragg grating on theemitting side may preferably be 800 μm or smaller and more preferably be700 μm or smaller. Further, on the viewpoint of improving the output ofthe laser, the length of the resonator may preferably be 300 μm orlarger.

According to each of the examples described above, the optical waveguideis a ridge type optical waveguide including a ridge portion and at leasta pair of ridge grooves defining the ridge portion. In this case, anoptical material is left under the ridge grooves, and elongated portionsof the optical material are also left on the outside of the ridgegrooves, respectively.

However, in the ridge type optical waveguide, it is possible to form anelongate and stripe-shaped core, by removing the optical material underthe ridge grooves. In this case, the ridge type optical waveguide iscomposed of an elongate core of an optical material, and the crosssection of the core is defined by a convex shape. A buffer layer (cladlayer) or air layer is present around the core and functions as a clad.

The convex shape means that line segments each connecting optional twopoints on an outer profile line of the cross section of the core arepresent inside of the outer profile line of the cross section of thecore. Such figure includes polygons such as triangle, quadrilateral,hexagon. heptagon or the like, circle, ellipse or the like.Quadrilateral may preferably be that having an upper side, a lower sideand a pair of side lines, and more preferably be trapezoid.

FIGS. 11 and 12 relate to this embodiment.

According to a grating device 21A of FIG. 11(a), a buffer layer 16 isformed on a supporting substrate 10, and an optical waveguide 30 isformed on the buffer layer 16. The optical waveguide 30 is composed of acore composed of the optical material having a refractive index of 1.8or higher. The shape of the cross section (cross section in thedirection perpendicular to the propagating direction of light) of theoptical waveguide is trapezoid, and the optical waveguide is extendedand elongated. According to the present example, the upper side face ofthe optical waveguide 30 is narrower than the lower side face. Theincident side propagating portion, Bragg grating and emitting sidepropagating portion as described above are formed in the opticalwaveguide 30.

According to a grating device 21B of FIG. 11(b), a buffer layer 22 isformed on the supporting substrate 10, and the optical waveguide 30 isembedded inside of the buffer layer 22. The shape of the cross section(cross section in the direction perpendicular to the propagatingdirection of light) of the optical waveguide is trapezoid, and theoptical waveguide is extended and elongated. According to the presentexample, the upper side face is narrower than the lower side face of theoptical waveguide 30. The buffer layer 22 includes an upper side buffer22 b and a lower side buffer 22 b of the optical waveguide 30 and sideface buffers 22 c covering side faces of the optical waveguide 30.

According to a grating device 21C of FIG. 11(c), a buffer layer 22 isformed on the supporting substrate 10, and the optical waveguide 30A isembedded inside of the buffer layer 22. The optical waveguide 30A iscomposed of the core composed of the optical material having arefractive index of 1.8 or higher. The shape of the cross section (crosssection in the direction perpendicular to the propagating direction oflight) of the optical waveguide is trapezoid, and the optical waveguideis extended and elongated. According to the present example, the lowerside face of the optical waveguide 30A is narrower than the upper sideface.

According to a grating device 21D of FIG. 12(a), a buffer layer 16 isformed on the supporting substrate 10, and the optical waveguide 30 isformed on the buffer layer 16. Then, the optical waveguide 30 issurrounded by and embedded in another buffer layer 23. The buffer layer23 includes an upper side buffer 23 a and side face buffers 23 b.According to the present example, the upper side face of the opticalwaveguide 30 is narrower than the lower side face.

According to a grating device 21E of FIG. 12(b), a buffer layer 16 isformed on the supporting substrate 10, and the optical waveguide 30A isformed on the buffer layer 16. Then, the optical waveguide 30A issurrounded by and embedded in another buffer layer 23. The buffer layer23 includes an upper side buffer 23 a and side face buffers 23 b.According to the present example, the lower side face of the opticalwaveguide 30A is narrower than the upper side face.

Then, the width Wm of the optical waveguide is defined as a width of thenarrowest part in the cross section of the optical waveguide.

EXAMPLES Inventive Example 1

It was fabricated the system shown in FIGS. 1 to 3.

Specifically, Ti film was formed on a substrate composed of z-plate oflithium niobate crystal doped with MgO, and photolithography techniquewas utilized to produce grating pattern in the direction of y-axis.Thereafter, fluorine-based reactive ion etching was performed using theTi pattern as a mask to form grating grooves at a pitch spacing Λ of 180nm and a length Lb of 100 μm. The depth of the grating groove was 300nm. Further, for forming the optical waveguide for propagation iny-axis, the grooves each having a width Wm of 3 μm and Tr of 0.5 μm wereprocessed in the grating portion by means of excimer laser. Further, thebuffer layer 17 made of SiO₂ and of 0.59 μm was formed by a sputteringsystem on the face with the grooves formed thereon, and a black LNsubstrate was used as the supporting body to adhere the face with thegrating formed thereon.

Then, the black LN substrate was adhered onto a polishing surface plateand the back face of the LN substrate with the grating was subjected toprecision polishing to a thickness (Ts) of 1 μm. Thereafter, it wasremoved off from the surface plate and the buffer layer 16 composed ofSiO₂ and of 0.5 μm was formed on the polished face by sputtering.

Thereafter, the assembly was cut into bars by means of a dicingequipment and both end faces of the bar was subjected to opticalpolishing. AR coatings of 0.1% or lower were formed on the both endfaces, respectively, and the bar was cut into chips to produce thegrating devices. The size of the device was a width of 1 mm and a lengthLWG of 500 μm.

As to the optical characteristics of the grating device, a superluminescence diode (SLD), a light source for wide band wavelength, wasused to input light into the grating device and its output light wasanalyzed by an optical spectrum analyzer to evaluate the reflectioncharacteristics based on the transmitting characteristics. As a result,it was obtained the characteristics that the central wavelength was 800nm, maximum reflectance was 3 percent and full width at half maximumΔλ_(G) was 1.3 nm with respect to polarized light (ordinary light) inthe direction of x axis.

Then, for evaluating the characteristics of the external resonator laserutilizing the grating device, the laser module was mounted as shown inFIG. 1. As the light source device, it was prepared one having a GaAsseries laser structure, in which a high refractive index film wasprovided on its one end face and an AR coating of a reflectance of 0.1%was provided on the other end face.

Specification of the Light Source Device:

Central wavelength; 800 nm

Length of laser device; 250 μm

Specification of Mounting

Lg: 3 μm

Lm; 20 μm

After mounting the module, the device was driven under current control(ACC) without utilizing a Peltier device to obtain the lasercharacteristics of a central wavelength of 800 nm and an output power of50 mW. Further, the module was set in a thermostatic bath for evaluatingthe operational temperature range to measure the temperature dependencyof the laser oscillating wavelength, the temperature with the modehopping occurred and the deviation of output power. As a result, thetemperature coefficient of the oscillating wavelength was 0.05 nm/° C.,the temperature of the mode hopping was 60° C., and the deviation of theoutput power was within 1 percent (FIGS. 5 and 7).

Comparative Example 1

As the Inventive Example 1, Ti film was formed on a substrate composedof z-plate of lithium niobate crystal doped with MgO, andphotolithography technique was utilized to produce grating pattern inthe direction of y-axis. Thereafter, fluorine-based reactive ion etchingwas performed using the Ti pattern as a mask to form grating grooves ata pitch spacing Λ of 180 nm and a length Lb of 1000 μm. The depth of thegrating grooves was 300 nm. Further, for forming the optical waveguidefor propagation in y-axis, the grooves each having a width Wm of 3 μmand Tr of 0.5 μm were processed in the grating portion by means ofexcimer laser.

Further, the buffer layer 17 made of SiO₂ and of 0.5 μm was formed by asputtering system on the side with the grooves formed thereon, and ablack LN substrate was used as the supporting body to adhere the facewith the grating formed thereon.

Then, the black LN substrate was adhered onto a polishing surface plateand the back face of the LN substrate with the grating was subjected toprecision polishing to a thickness (Ts) of 1 μm. Thereafter, it wasremoved off from the surface plate and the buffer layer 16 composed ofSiO₂ and of 0.5 μm was formed on the polished face by sputtering.Thereafter, the assembly was cut into bars by means of a dicingequipment and both end faces of the bar was subjected to opticalpolishing. AR coatings of 0.1% or lower were formed on the both endfaces, respectively, and the bar was cut into chips to produce thegrating devices. The size of the device was a width of 1 mm and a lengthLWG of 1500 μm.

As to the optical characteristics of the grating device, a superluminescence diode (SLD), a light source for wide band wavelength, wasused to input light into the grating device and its output light wasanalyzed by an optical spectrum analyzer to evaluate the reflectioncharacteristics based on the transmitting characteristics. As a result,it was obtained the characteristics that the central wavelength was 800nm, maximum reflectance was 10 percent and full-width at half maximumΔλ_(G) was 0.2 nm with respect to polarized light (ordinary light) inthe direction of x axis.

Then, for evaluating the characteristics of the external resonator laserutilizing the grating device, the laser module was mounted as shown in aseparate figure. As the light source device, it was prepared one havinga GaAs series laser structure, in which a high refractive index film wasprovided on its one end face and an AR coating of a reflectance of 0.1%was provided on the other end face.

Specification of the Light Source Device:

Central wavelength; 800 nm

Length of laser device; 1000 μm

Specification of Mounting

Lg: 3 μm

Lm; 20 μm

After mounting the module, the device was driven under current control(ACC) without utilizing a Pertier device to obtain the lasercharacteristics of a central wavelength of 800 nm and an output power of50 mW. Further, the module was set in a thermostatic bath for evaluatingthe operational temperature range to measure the temperature dependencyof the laser oscillating wavelength, the temperature with the modehopping occurred and the deviation of output power. As a result, thetemperature coefficient of the oscillating wavelength was 0.05 nm/° C.,the temperature of the mode hopping was 6° C., and the deviation of theoutput power was 10 percent.

Inventive Example 2

It was fabricated the system shown in FIGS. 1 and 4.

Specifically, Ti film was formed on a substrate composed of z-plate oflithium niobate crystal doped with MgO, and photolithography techniquewas utilized to produce grating pattern in the direction of y-axis.Thereafter, fluorine-based reactive ion etching was performed using theTi pattern as a mask to form grating grooves at a pitch spacing Λ of 214nm and a length Lb of 100 μm. The depth of the grating groove was 40 nm.Further, for forming the optical waveguide for propagation in y-axis,the grooves each having a width Wm of 3 μm and Tr of 0.5 μm wereprocessed in the grating portion by means of excimer laser. Further, thebuffer layer 17 made of SiO₂ and of 0.5 μm was formed by a sputteringsystem on the face with the grooves formed thereon, and a black LNsubstrate was used as the supporting body to adhere the face with thegrating formed thereon. The black LN means lithium niobate whose oxygencontent is depleted so that generation of electric charges due topyroelectricity can be prevented. It is thus possible to prevent cracksof the substrate due to surge resistance responsive to temperaturechange.

Then, the supporting body was adhered onto a polishing surface plate andthe back face of the supporting body with the grating was subjected toprecision polishing to a thickness (Ts) of 1 μm. Thereafter, it wasremoved off from the surface plate and the buffer layer 16 composed ofSiO₂ and of 0.5 μm was formed on the polished face by sputtering.

Thereafter, the assembly was cut into bars by means of a dicingequipment and both end faces of the bar were subjected to opticalpolishing. AR coatings of 0.1% or lower were formed on the both endfaces, respectively, and the bar was cut into chips to produce thegrating devices. The size of the device was a width of 1 mm and a lengthLWG of 500 μm.

As to the optical characteristics of the grating device, a superluminescence diode (SLD), a light source for wide band wavelength, wasused to input light into the grating device and its output light wasanalyzed by an optical spectrum analyzer to evaluate the reflectioncharacteristics based on the transmitting characteristics. As a result,it was obtained the characteristics that the central wavelength was 945nm, maximum reflectance was 20 percent and full width at half maximumΔλ_(G) was 2 nm with respect to TE mode.

Then, for evaluating the characteristics of the external resonator laserutilizing the grating device, the laser module was mounted as shown inFIG. 1. As the light source device, it was prepared one having a GaAsseries laser structure, in which a high refractive index film wasprovided on its one end face and an AR coating of a reflectance of 0.1%was provided on the other end face.

Specification of the Light Source Device:

Central wavelength; 950 nm

Output power: 20 mW

Half value width: 50 nm

Length of laser device; 250 μm

Specification of Mounting

Lg: 1 μm

Lm; 20 μm

After mounting the module, the device was driven under current control(ACC) without utilizing a Peltier device to obtain the lasercharacteristics of a central wavelength of 945 nm and an output power of50 mW. FIG. 8 shows the characteristic spectrum of the laser. Further,the module was set in a thermostatic bath for evaluating the operationaltemperature range to measure the temperature dependency of the laseroscillating wavelength and the deviation of output power. As a result,the temperature coefficient of the oscillating wavelength was 0.05 nm/°C., the temperature range with the large deviation of output power inthe temperature range due to the mode hopping was 80° C., and thedeviation of the output power was within 1 percent even in the case thatthe mode hopping took place.

Inventive Example 3

It was formed the grating grooves at a pitch spacing Λ of 222 nm and alength Lb of 100 μm, according to the same procedure as the example 2.The depth of the grating groove was 40 nm. As to the opticalcharacteristics of the grating device, a super luminescence diode (SLD),a light source for wide band wavelength, was used to input light intothe grating device and its output light was analyzed by an opticalspectrum analyzer to evaluate the reflection characteristics based onthe transmitting characteristics. As a result, it was obtained thecharacteristics that the central wavelength was 975 nm, maximumreflectance was 20 percent and full width at half maximum Δλ_(G) was 2nm with respect to TE mode.

Then, the laser module was mounted as shown in FIG. 1. As the lightsource device, it was applied a conventional GaAs series laser whoseemitting face was not covered with an AR coating.

Specification of the Light Source Device:

Central wavelength; 977 nm

Output power: 50 mW

Half value width: 0.1 nm

Length of laser device; 250 μm

Specification of Mounting

Lg: 1 μm

Lm; 20 μm

After mounting the module, the device was driven under current control(ACC) without utilizing a Peltier device to obtain the lasercharacteristics that it was oscillated at a central wavelength of 975 nmresponsive to the reflection wavelength of the grating and the outputpower was 40 mW, although the output power was smaller than thatobtained in the case that the grating device is not provided. Further,the module was set in a thermostatic bath for evaluating the operationaltemperature range to measure the temperature dependency of the laseroscillating wavelength and the deviation of output power. As a result,the temperature coefficient of the oscillating wavelength was 0.05 nm/°C., the temperature with the large deviation of output power in thetemperature range due to the mode hopping was 80° C., and the deviationof the output power was within 1 percent even in the case that the modehopping took place.

Comparative Example 2

In the case that the grating device is not provided in the inventiveexample 3, the temperature coefficient of the laser oscillatingwavelength was as large as 0.3 nm/° C. and the mode hopping temperaturewas about 10° C. The deviation of the power was large and the deviationof output power was larger than 10 percent, at 10° C. or higher.

Inventive Example 4

Ta₂O₅ film was formed on a quartz substrate for 1.2 μm by a sputteringsystem to provide the optical material layer. Then, Ti film was formedon Ta₂O₅ and photolithography technique was utilized to produce gratingpattern in the direction of y-axis. Thereafter, fluorine-based reactiveion etching was performed using the Ti pattern as a mask to form gratinggrooves at a pitch spacing Λ of 232 nm and a length Lb of 100 μm. Thedepth of the grating groove was 40 nm. Further, it was produced theoptical waveguide having the shape shown in FIGS. 2 and 3 of thespecification, by the reactive ion etching according to the sameprocedure described above. As to the optical characteristics of thegrating device, a super luminescence diode (SLD), a light source forwide band wavelength, was used to input light into the grating deviceand its output light was analyzed by an optical spectrum analyzer toevaluate the reflection characteristics based on the transmittingcharacteristics. As a result, it was obtained the characteristics thatthe central wavelength was 945 nm, maximum reflectance was 20 percentand full width at half maximum Δλ_(G) was 2 nm with respect to TE mode.

Then, the laser module was mounted as shown in FIG. 1. As the lightsource device, it was applied a conventional GaAs series laser with a0.1% AR coating formed on the emitting face.

Specification of the Light Source Device:

Central wavelength; 950 nm

Output power: 20 mW

Half value width: 50 nm

Length of laser device; 250 μm

Specification of Mounting

Lg: 1 μm

Lm; 20 μm

After mounting the module, the device was driven under current control(ACC) without utilizing a Peltier device to obtain the lasercharacteristics that it was oscillated at a central wavelength of 945 nmresponsive to the reflection wavelength of the grating and that theoutput power was 40 mW, although the output power was smaller than thatobtained in the case that the grating device is not provided. Further,the module was set in a thermostatic bath for evaluating the operationaltemperature range to measure the temperature dependency of the laseroscillating wavelength and the deviation of output power. As a result,the temperature coefficient of the oscillating wavelength was 0.03 nm/°C., the temperature range with the large deviation of output power dueto the mode hopping was 50° C., and the deviation of the output power inthe temperature range was within 1 percent even in the case that themode hopping took place.

Inventive Example 5

It was produced the system shown in FIGS. 1 and 3, according to the sameprocedure as the Example 1. However, the cross sectional shape of thegrating device 21D was made as shown in FIG. 12(a).

Specifically, an SiO₂ layer 16 forming the lower clad layer was formedon a supporting substrate 10 made of quartz in a thickness of 0.5 μm bya sputtering system, and Ta₂O₅ was formed thereon for 1.29 μm to providethe optical material layer.

Then, Ti film was formed on Ta₂O₅ and grating pattern was produced by anED drawing system. Thereafter, fluorine-based reactive ion etching wasperformed using the Ti pattern as a mask to form grating grooves at apitch spacing Λ of 238.5 nm and a length Lb of 100 μm. The depth of thegrating groove was 40 nm.

Further, for forming the optical waveguide 30, refractive ion etchingwas performed according to the same procedure as described above tocompletely cut the optical layer to leave the optical waveguide of awidth Wm of 3 μm and the both sides. The thickness Ts of the opticalwaveguide 30 was made 1.2 μm.

Finally, the buffer layer 23 made of SiO₂ was formed as the upper cladin 2 μm by sputtering to cover the optical waveguide 30.

Thereafter, the assembly was cut into bars by means of a dicingequipment and both end faces of the bar was subjected to opticalpolishing. AR coatings of 0.1% or lower were formed on the both endfaces, respectively, and the bar was cut into chips to produce thegrating devices. The size of the device was a width of 1 mm and a lengthLWG of 500 μm.

As to the optical characteristics of the grating device, a superluminescence diode (SLD), a light source for wide band wavelength, wasused to input light of TE mode into the grating device and its outputlight was analyzed by an optical spectrum analyzer to evaluate thereflection characteristics based on the transmitting characteristics. Itwas obtained the characteristics that the central wavelength was 975 nm,reflectance was 18 percent and full width at half maximum Δλ_(G) was 2nm as to the thus measured grating device.

Then, the laser module was mounted as shown in FIG. 1. As the lightsource device, it was applied a conventional GaAs series laser withoutan AR coating on the emitting face.

Specification of the Light Source Device:

Central wavelength; 977 nm

Output power: 50 mW

Half value width: 0.1 nm

Length of laser device; 250 μm

Specification of Mounting

Lg: 1 μm

Lm; 20 μm

After mounting the module, the device was driven under current control(ACC) without utilizing a Peltier device to obtain the lasercharacteristics that it was oscillated at a central wavelength of 975 nmresponsive to the reflection wavelength of the grating and that theoutput power was 40 mW, although the output power was smaller than thatobtained in the case that the grating device is not provided. Further,the module was set in a thermostatic bath for evaluating the operationaltemperature range to measure the temperature dependency of the laseroscillating wavelength and the deviation of output power. As a result,the temperature coefficient of the oscillating wavelength was 0.03 nm/°C., the temperature range with the large deviation of output power dueto the mode hopping was 40° C., and the deviation of the output power inthe temperature range was within 1 percent even in the case that themode hopping took place.

1. An external resonator type light emitting system comprising a lightsource oscillating a semiconductor laser light by itself and a gratingdevice providing an external resonator with said light source, saidlight emitting system performing oscillation in single mode; whereinsaid light source comprises an active layer oscillating saidsemiconductor laser light; wherein said grating device comprises anoptical waveguide comprising an incident face to which saidsemiconductor laser light is incident and an emitting face of emittingan emitting light having a desired wavelength, a Bragg grating formed insaid optical waveguide, and a propagating portion provided between saidincident face and said Bragg grating; wherein the following formulas (1)to (5) are satisfied, wherein said light source and said grating deviceare optically connected to each other directly; wherein said externalresonator is constituted between said Bragg grating and an outer sideend face of said active layer on the opposite side to an emitting faceof said active layer; and wherein a length between said outer side endface of said active layer and an emitting side end point of said Bragggrating is 910 μm or smaller, wherein said optical waveguide comprises acore; and wherein a cross section of said core is of a convex shape.$\begin{matrix}{{\Delta \; \lambda_{G}} \geqq {0.8\mspace{14mu} {nm}}} & (1) \\{L_{b} \leqq {500\mspace{14mu} {µm}}} & (2) \\{L_{a} \leqq {500\mspace{14mu} {µm}}} & (3) \\{n_{b} \geqq 1.8} & (4) \\{{{\frac{\lambda_{G}}{T} - \frac{\lambda_{TM}}{T}}} \leqq {0.03\mspace{14mu} {nm}\text{/}{{{^\circ}C}.}}} & (5)\end{matrix}$ (Δλ_(G) represents a full width at half maximum of a peakof a Bragg reflectance in said formula (1). L_(b) represents a length ofsaid Bragg grating in said formula (2). L_(a) represents a length ofsaid active layer in said formula (3). n_(b) represents a refractiveindex of a material forming said Bragg grating in said formula (4).dλ_(G)/dT represents a temperature coefficient of a Bragg wavelength,and dλ_(TM)/dT represents a temperature coefficient of a wavelengthsatisfying a phase condition of an external resonator laser in saidformula (5).)
 2. The system of claim 1, wherein the following formulas(6) to (8) are satisfied.L _(WG)≦600 μm  (6)1 μm≦L _(g)≦10 μm  (7)20 μm≦L _(m)≦100 μm  (8) (L_(WG) represents a length of said gratingdevice in said formula (6). L_(g) represents a distance between anemitting face of said light source and said incident face of saidoptical waveguide in said formula (7). L_(m) represents a length of saidpropagating portion in said formula (8).)
 3. The system of claim 1,wherein a reflectance of said Bragg grating is higher than each ofreflectances at an emitting face of said light source, at said incidentface of said grating device and at said emitting face of said gratingdevice.
 4. The system of claim 2, wherein a reflectance of said Bragggrating is higher than each of reflectances at an emitting face of saidlight source, at said incident face of said grating device and at saidemitting face of said grating device.
 5. The system of claim 1, whereinsaid material of said Bragg grating is selected from the groupconsisting of gallium arsenide, lithium niobate single crystal, tantalumoxide, zinc oxide and aluminum oxide.
 6. The system of claim 2, whereinsaid material of said Bragg grating is selected from the groupconsisting of gallium arsenide, lithium niobate single crystal, tantalumoxide, zinc oxide and aluminum oxide.
 7. The system of claim 3, whereinsaid material of said Bragg grating is selected from the groupconsisting of gallium arsenide, lithium niobate single crystal, tantalumoxide, zinc oxide and aluminum oxide.
 8. The system of claim 1, furthercomprising a buffer layer provided on said optical waveguide.
 9. Thesystem of claim 2, further comprising a buffer layer provided on saidoptical waveguide.
 10. The system of claim 3, further comprising abuffer layer provided on said optical waveguide.
 11. The system of claim4, further comprising a buffer layer provided on said optical waveguide.